1. Industrial fields
This invention concerns a humidity sensor. The humidity sensor in this invention includes, besides an ordinary humidity sensor, a wide range of sensors applicable to various purposes such as dew condensation sensor, rain drop sensor etc.
2. Prior art
Among humidity sensors hitherto used, there are, for example, hair hygrometers and wet and dry bulb hygrometers which are in wide use for controlling/preventing dew condensation on anti-cloud glass used for autovehicles and aircrafts.
However, those hitherto in use need manual maintenance and moreover lack reliability. Therefore, their practical application is not satisfactory.
In view of this, recently, as can be seen in Japanese Patent Publication Sho 55-49705, humidity sensors using high molecular substances or ceramics as humidity sensing elements have been developed, some of which are already in practical use.
Conventional humidity sensors for which a high molecular substance is used as a humidity sensing element afford good sensitivity but have the following drawbacks: they are easily contaminated and their life is short. The one using ceramics is of good stability and capable of vaporizing contaminant particles by heating. ln contrast to these advantages, the disadvantages are high level of labor and manufacturing cost, poor adhesion between the element and base plate, poor sensitivity, complicated circuit configuration, short life of the sensor, slow response speed etc. Indeed, these drawbacks have up to now prevented a hygrometer or humidity sensor from propagating into use fields in general terms.
A complementary explanation on this point using FIG. 1A and FIG. 1B is as follows. The humidity sensing element 7, made of semiconductive ceramics, is manufactured by a high pressure press forming of a raw oxide powder mixture, which then is baked at a high temperature to a sintered compact.
This process requires the following stages; compounding of raw materials, press forming, and high temperature sintering. Meanwhile, the humidity sensing element to be coated on the base plate requires raw material compounding, flame spraying or printing, and baking. These stages not only require experience and high skill but costly production facilities have to be used such as a press machine, baking oven, flame spraying machine, printer and so on.
It is notable here that, as the electrically insulated base plate over which the humidity sensing element is to be coated, an alumina base plate or heat-resistant glass base plate is used.
As explained above, manufacturing a humidity sensing element is done by using a mixture of raw oxide powder and ceramics or heat-resistant glass base plate, which then requires complicated processing stages and costly manufacturing. The final manufacturing cost becomes very high.
This is the reason why the use of humidity sensors has not been more widespread.
The humidity sensing element 1, manufactured this way is of high electric resistance or impedance, a parameter corresponding to humidity. Therefore, in order to determine the resistance by making a connection with an outside electronic circuit, a device is made to reduce inter-electrode resistance by giving opposing electrodes 2 and 2' a comb-like structure, owing to which their opposing portions are given more length. These comb-like electrodes are connected with the terminal electrodes 3 and 3': 3 and 3' are connected respectively with lead wires 4 and 4' to be connected with the outside electronic circuit, wherein the connection is done by soldering at electrode terminal posts 5 and 5' or by electric welding 8 and 8'.
The comb-like structured opposing electrodes 2 and 2' are prepared by print-application of gold paste or ruthenium oxide paste on humidity-sensing element 1 or base plate 7, followed by firing. However, due to the current printing technology and paste spreading, it is hard to maintain a prescribed interelectrode distance. Where this distance is less than 0.3 mm, there is the possibility of a short-circuit between opposing electrodes.
Accordingly, when the application of high price paste and a printing technology is nevertheless unable to provide the interelectrode resistance with the required value for the electronic circuit design, it is necessary to make the resistance smaller by increasing the number of electrodes or by making the electrode length greater, whereby the sensor has been restricted by designing/minimizing the shape.
Further, in order to have a linkage between the sensor and the electronic circuit by connecting lead frames or lead wires 4 and 4' with electrode terminals 3 and 3', electrode terminal pastes 5 and 5', previously printed/baked with silver or gold paste, are provided. When such portions are soldered, the flux used there often spatters and penetrates into the humidity sensing element, giving rise to a bad effect on the performance thereof. To avoid such contaminations, designing certain countermeasures in advance to protect the humidity sensing element 1 from contamination or taking necessary measures to remove contaminants after soldering, should be done. Any way, burdensome steps are required, which is a cause of lowered productivity and quality.
In the case of soldering at 8 and 8' for the connection, the problem of flux does not exist. However, due to the difficulty of inspecting whether or not the connection is done perfectly, poor circuit continuity might cause trouble after making.
Consequently, in any case, the stage of connecting the sensor electrode terminal 3 and 3' with lead frame or lead wire 4 and 4' is of prime importance as it might affect the performance of the sensor by reducing productivity and yield.
As explained above re the humidity sensor hitherto in use, the humidity sensing element thereof requires high manufacturing cost. Further, in view of the high electric resistance of this element, the device is made so as to alleviate this difficulty by giving opposing electrodes a comb-like structure. This structure also needs costly materials and high technology. Consequently, the manufacturing cost is raised and minimizing the sensor size is hindered. Moreover, the necessary treatment of electrode terminals for connecting the sensor and the outside electronic circuit often cause the sensing performance to deteriorate, and productivity and yield are lowered, thereby again causing the manufacturing cost to rise.
The purpose of this invention is to provide a solution to the problems described above, which exist in the technology hitherto in use in relation to humidity sensors, including the manufacturing method for the sensor and sensing element.